Although aging progresses in organs throughout the body, its effects can be observed particularly in the skin, and with respect to the face in particular where attention tends to be easily focused, wrinkles and crow's feet that occur with aging cause concern for many middle-aged and elderly persons, and especially women. There has conventionally been a strong demand for cosmetics for improving such wrinkles, and although various efforts have been made thus far in response thereto, since there are many aspects of the mechanism relating to aging and wrinkles that remain unclear, an invention yielding satisfactory effects has yet to be realized. On the other hand, since retention of moisture by the outermost layer of the epidermis known as the corneal layer keeps the skin flexible, gives it elasticity and protects the dermis, certain objectives were expected to be achieved.
Amidst these circumstances, compositions for external application to skin have conventionally incorporated moisturizers for the purpose of improving usability and safety and preventing putrefaction and the like in addition to demonstrating moisturizing effects, examples of which include polyols and polyethers of glycerin, 1,3-butylene glycol, xylitol, sorbitol, erythritol, maltitol, polyethylene glycol, propylene glycol or diglycerin-(EO)/(PO) addition products and the like. Although these substances have moisturizing effects, their effects for preventing and/or improving wrinkles are inadequate, while further having problems with usability such as stickiness as well as problems such as the generation of odors as a result of oxidation.
In addition, polymer compounds of hyaluronic acid, mucoitin sulfate, trichosanic acid, chondroitin sulfate, soluble collagen or atherocollagen and the like are also used as moisturizers. Although these have higher moisturizing effects than polyols and the like, their moisturizing effects are still inadequate for preventing and/or improving wrinkles. In addition, since their effects are based on moisture retention by the skin's surrounding environment, polymer compounds have inferior penetrability into the skin and do not act directly within skin. In addition, these polymer compounds also have problems of requiring excessive time for neutralization and dissolution during their incorporation, as well as problems with usability in terms of causing tightness by creating the sensation of a coating on the skin and being even stickier than polyols.
Examples of low molecular weight compounds other than polyols widely used as moisturizers include sodium lactate, bile acid salts, pyrrolidone carboxylates and amino acids. Since these are low molecular weight compounds, although they have the potential to demonstrate their own functions by penetrating into the corneal layer, their own moisturizing effects are not that high and are inadequate for preventing and/or improving wrinkles. Sarcosine is an example of a moisturizer that that improves the moisture retention and moisture absorption properties of the compound itself (Patent Document 1). Although this compound itself has moisture absorbing properties, since it does not act directly on moisture retention in the skin (moisture in the corneal layer), moisturizing effects are inadequate for preventing and/or improving wrinkles. Moreover, since these compounds, including sarcosine, have high crystallinity and low solubility in substances other than water, when incorporated in compositions for external application to skin having a low moisture content, they end up precipitating with the evaporation of moisture following their application, thereby resulting in the problems of having inferior sustained moisturizing effects as well as inhibiting penetration into the corneal layer. Moreover, not only is it difficult to incorporate large amounts of these compounds in preparations such as milky lotions and creams containing low amounts of water, since the solubility of these compounds is inferior even in ethanol, they also have the problem of inferior low-temperature stability in cosmetic lotions having a high ethanol content.
As a result of conducting extensive research in consideration of the aforementioned circumstances on substances that have moisturizing effects to a degree to which wrinkle preventing and/or improving effects are expected, do not precipitate following application of a composition for application to skin, have superior penetrability, increase the moisture content in the corneal layer by acting directly on the corneal layer, and result in superior stability for the composition for application to skin in which they are incorporated, the inventors of the present invention found that aminoacetoamide compounds of the present invention and salts thereof demonstrate superior moisturizing effects, have high penetrability, increase moisture content in the corneal layer by acting directly on the corneal layer, have superior anti-wrinkle effects, have superior water solubility, and also have superior solubility in substances other than water, while also facilitating preparation of compositions for external application to skin and resulting in superior stability of the prepared compositions for external application to skin, thereby leading to completion of the present invention.
Patent Document 1: Japanese Patent Publication No. 3441387